The present invention relates to a bearing with improved frictional behavior and, more particularly, to a bearing having a load-bearing surface whose load-carrying capacity is improved by the presence of micropores.
It is well known from the theory of hydrodynamic lubrication that when two parallel surfaces, separated by a lubricating film, slide at some relative speed with respect to each other, no hydrodynamic pressure, and hence no separating force, can be generated in the lubricating film. The mechanism for hydrodynamic pressure buildup requires a converging film thickness in the direction of sliding. In conventional applications, this often is obtained by some form of misalignment or eccentricity between the sliding surfaces, for example, hydrodynamic thrust and journal bearings. Macrosurface structure, particularly in the form of waviness on the sliding surfaces, has been studied in the past for both parallel face thrust bearings and mechanical seals. The load carrying capacity in these cases is due to an asymmetric hydrodynamic pressure distribution over the wavy surface. The pressure increase in the converging film regions is much larger than the pressure drop in the diverging film regions. This is because the pressure drop is bounded from below by cavitation, whereas the pressure increase has effectively no upper limit. Microsurface structure in the form of protruding microasperities on the sliding surfaces also can be used to generate a locally asymmetric pressure distribution with local cavitation. The integrated effect of these microasperities can be useful in producing separating force between parallel sliding surfaces. Asymmetric pressure distribution also can be obtained by depressed surface structures, and various forms of grooves are used in bearings and mechanical seals. See, for example, T. W. Lai, "Development of Non-Contacting, Non-Leaking Spiral Groove Liquid Face Seals", Lubr. Eng. vol. 50 pp. 625-640 (1994).
Microsurface structure in the form of micropores would have several advantages over other microsurface structures, particularly those involving protruding structures, in moving load-bearing surfaces. These advantages include:
1. Ease of manufacturing.
2. The ability to optimize pore size, shape, and distribution using theoretical models.
3. Good scaling capability in stationary machinery.
4. Providing microreservoirs for lubricant under starved lubrication conditions, for example, at startup and after lubricant loss.
5. Providing the capacity to sequester small wear debris.
There is thus a widely recognized need for, and it would be highly advantageous to have, bearings with micropore structure in their load-bearing surfaces and a method for designing the distribution and geometry of the micropores.